The other night I was on WN602 MDW-BNA. We passed some impressive storms along the way. I went to check flightaware to see what the flight plan was and our flight number was changed to WN9018. http://flightaware.com/live/flight/SWA9018

However, the fight continued as WN602 from BNA-AUS according to flightaware. Why was the one leg changed to 9018? Arent the 9000 flight numbers normally for a ferry flight?